1. Field of the Invention
The present invention relates to agricultural product tanks, and more particularly to agricultural product tanks for containing material under pressure.
2. Description of Related Art
Air seeders, sprayers and other similar types of agricultural implements typically include product tanks for bulk storage of material to be distributed by the implement onto or underneath the soil. Those tanks designed for storing and distributing granular material generally include a lower, funnel portion with an angled design to efficiently dispense the granular material, and an upper portion which is generally larger in volume than the funnel portion for increasing storage capacity. These tanks have been fabricated by welding and/or bolting sheet metal panels in a tank configuration on an implement frame. Sheet metal construction is relatively expensive, primarily because a relatively long fabrication time is required to assemble the panels on the support frame. In addition, these metal tanks are relatively heavy and subject to water leakage and corrosion, particularly in tanks which are used to contain fertilizer.
As an alternative to metallic fabrication, the more recent past has seen a progression toward manufacture of such tanks from composites or plastics. These plastic tanks do not corrode as a result of the products transported within the tanks. Also, the plastic tanks overcome most of the water leakage problems attendant metallic construction. In addition, the plastic tanks are generally less expensive to make and weigh less than tanks made from steel.
While plastic product tanks eliminate some of the disadvantages of metallic tanks, plastic tanks heretofore available present another set of problems. For instance, where the tanks are subjected to air pressure as, for example, in a positive pressure air seeding system, a substantial bulging effect is imparted to the surface of the upper portion of the tank. Conventional tanks are known which are generally octagonal when viewed from the side, but which are generally rectangular in plan. These tanks are made up of fairly large panels joined at their edges so that the middle regions of the panels receive insufficient support to withstand pressure and therefore tend to bulge excessively.
Some known plastic tanks have a rounded surface or surfaces in their upper portions, allowing the tank to more easily withstand air pressure because the pressure is more evenly distributed. During periods where such a tank is unpressurized, however, portions of the rounded surfaces tend to become concave (i.e., "cave in"), under their own weight or when they come in contact with the operator or with equipment such as an on-board auger. The amount of air pressure or force due to the weight of product within the tank may not be enough to restore the outwardly convex shape. Over time, the plastic tanks tend to develop a "memory" for the undesirable concave shape and the deformity will remain permanent.
In addition, repeated flexing of the tank weakens the material from which it is constructed and makes it more prone to cracking and failure. When the tanks are unpressurized when loading product, the product itself may be unable to push walls out without the assistance of air pressure. Therefore, the tank may not be filled to nominal capacity. Also, a tank with collapsed walls is not aesthetically appealing.
Conventional tanks typically have four sides in plan and at least 4 corners with interior angles of about 90.degree.. Because these angles are relatively small, the tank must be made thicker to withstand resulting high stress concentrations. The increased thickness represents an increase in cost and weight.
Tanks with irregular shapes have been used, but still have relatively small interior angles between panels. U.S. Pat. No. 4,541,549, the disclosure of which is hereby incorporated by reference, describes such a plastic product tank which has eight sides in plan, yet retains several right angles at joints between side panels.